Abstract

Introduction: The JAK2 inhibitor ruxolitinib (RUX) is an inhibitor of the Janus kinase family of protein tyrosine kinases (JAKs) that is effective for the treatment of myeloproliferative diseases. Immunomodulatory drugs (IMiDs) including lenalidomide (LEN) and corticosteroids have shown efficacy for the treatment of multiple myeloma (MM). The JAK-STAT signaling pathway plays key roles in the growth and survival of malignant plasma cells in MM. In this study, we evaluated the preclinical anti-MM effects of RUX in combination with LEN and corticosteroids, both in vitro and in vivo, and in a patient with MM and polycythemia rubra vera (PRV).

Methods: The human MM cell lines U266, RPMI8226 and MM1S cells were derived from ATCC. Primary MM tumor cells were isolated from MM patients’ bone marrow aspirates. The cells were seeded at105 cells/100ul/well in 96-well plates and incubated for 24 h in the presence of vehicle, RUX, LEN or dexamethasone (DEX) alone, RUX + LEN, RUX + DEX, or all three drugs together for 48 h. Cell viability was quantified using the MTS cell proliferation assay. In vitro, synergy between ruxolitinib and lenalidomide or dexamethasone was assessed using the median effect method of Chou and Talalay. For the in vivo studies, the human myeloma tumors (LAGκ-1A or LAGκ-2) were surgically implanted into the left superficial gluteal muscle of anaesthetized naive SCID mice. Mice were blindly assigned to one of the experimental groups, and treatment was initiated 7–21 d after tumor implantation. LEN was administered via oral gavage daily (30 mg/kg). RUX (3 mg/kg) was given via intraperitoneal (IP) injection twice daily. Dexamethasone was administered daily (1.5mg/kg) via IP injection. An 88 year old MM patient with PRV who developed MM on RUX alone and then progressed on LEN+DEX was treated with the combination of all three drugs.

Results:In vitro, RUX induced concentration-dependent inhibition of viability in all three MM cell lines (U266, RPMI8226 and MM1S) at RUX 50 mM and inhibition of primary MM tumor cells at a higher concentration (100 mM). In contrast, RUX had negligible cytotoxic effects on normal peripheral blood mononuclear cells (PBMCs). We next examined cell viability in the presence of RUX plus LEN or DEX. First, U266 cells were incubated with a fixed concentration of LEN (30 mM) or DEX (40 mM) with increasing concentrations of RUX (0.1–100 mM) for 48 h. At RUX 50 mM, the cytotoxic effects of LEN were enhanced and at RUX 1 mM, the anti-myeloma effect of DEX was increased. Moreover, the cytotoxic effects of RUX, LEN and DEX were greater than RUX in combination with either LEN or DEX in U266 cells. Similar results were obtained using the RPMI8226 and MM1S cell lines as well as primary MM tumor cells. Next, we evaluated RUX in combination with lenalidomide and dexamethasone in vivo using SCID mice bearing either the human LAGκ-1A or LAGκ-2 MM xenografts. RUX (3mg/kg), LEN (15mg/kg) or DEX (1mg/kg) alone did not inhibit tumor growth in either mice bearing LAGκ-1A or LAGκ-2. In contrast, the combination of RUX with DEX but not LEN slightly decreased tumor volume. However, the combination of all three drugs at the same doses showed a marked reduction of tumor size and delay of tumor growth in both human MM xenograft models. In addition, a patient with MM and PRV experienced sustained and ongoing reductions in his serum M-protein, IgG, and 24-urine M-protein with achievement of a partial response on low doses of RUX (2.5 mg twice daily), LEN (2.5 mg daily), and methylprednisolone (20 mg daily) that has been ongoing for more than 12 months after developing MM on RUX alone and then progressing on the combination of LEN and methylprednisolone.

Conclusion: This study illustrates that the combination of the JAK2 inhibitor RUX, LEN and corticosteroids shows both preclinical and promising clinical results for the treatment of MM.